Hydroflux Access to Hydroxide‐Free Yttrium Oxovanadate and Arsenite Layers: K 6 [Y(VO 4 ) 3 ]·2H 2 O, K 6 [Y(VO 4 ) 3 ], and Na 3 [Y(AsO 3 ) 2 ]

Hydroflux synthesis provides access to crystalline rare‐earth compounds from highly concentrated alkaline media at comparatively low temperatures. Under these conditions, hydroxide coordination commonly dominates, and many rare‐earth hydroflux products are hydroxides, hydroxometalates, or mixed oxo‐hydroxo compounds. The yttrium compounds reported here represent a different structural outcome: highly connected oxometalate frameworks in which hydroxide ligands are absent from the Y 3+ coordination sphere. K 6 [Y(VO 4 ) 3 ]·2H 2 O crystallizes from KOH hydroflux as a layered oxovanadate (V) containing honeycomb‐type Y(VO 4 ) 3 ] 6– sheets, in which [YO 6 ] octahedra are linked by corner‐sharing [VO 4 ] tetrahedra; K + cations and water molecules occupy the interlayer region. Thermal dehydration yields K 6 [Y(VO 4 ) 3 ] with retention of the primary Y─O─V connectivity, while the layer stacking and potassium coordination are reorganized. Na 3 [Y(AsO 3 ) 2 ] is obtained from NaOH hydroflux and contains Y(AsO 3 ) 2 ] 3– layers built from corner‐sharing [YO 6 ] octahedra and trigonal‐pyramidal [AsO 3 ] groups. These compounds show that highly charged oxoanions can complete the coordination sphere of Y 3+ cations and suppress hydroxide incorporation in hydroflux media. Eu 3+ ‐doped K 6 [Y(VO 4 ) 3 ] 2H 2 O displays red emission under UV irradiation, providing qualitative evidence that the hydroxide‐free oxovanadate framework can accommodate luminescent rare‐earth cations as optically active centers.